[Focal][PATCH 2/3] x86: Introduce ia32_enabled()

[Yuxuan Luo]

From: Nikolay Borisov <nik.borisov at suse.com>

IA32 support on 64bit kernels depends on whether CONFIG_IA32_EMULATION
is selected or not. As it is a compile time option it doesn't
provide the flexibility to have distributions set their own policy for
IA32 support and give the user the flexibility to override it.

As a first step introduce ia32_enabled() which abstracts whether IA32
compat is turned on or off. Upcoming patches will implement
the ability to set IA32 compat state at boot time.

Signed-off-by: Nikolay Borisov <nik.borisov at suse.com>
Signed-off-by: Thomas Gleixner <tglx at linutronix.de>
Link: https://lore.kernel.org/r/20230623111409.3047467-2-nik.borisov@suse.com

(backported from commit 1da5c9bc119d3a749b519596b93f9b2667e93c4a)
[yuxuan.luo: ignored context conflicts and backported the changes.]
CVE-2024-25744
Signed-off-by: Yuxuan Luo <yuxuan.luo at canonical.com>
---
 arch/x86/entry/common.c     |  5 +++++
 arch/x86/include/asm/ia32.h | 16 +++++++++++++++-
 2 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
index 721109c0cf994..1aa1c302f392f 100644
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -303,6 +303,11 @@ __visible void do_syscall_64(unsigned long nr, struct pt_regs *regs)
 #endif
 
 #if defined(CONFIG_X86_32) || defined(CONFIG_IA32_EMULATION)
+
+#ifdef CONFIG_IA32_EMULATION
+bool __ia32_enabled __ro_after_init = true;
+#endif
+
 /*
  * Does a 32-bit syscall.  Called with IRQs on in CONTEXT_KERNEL.  Does
  * all entry and exit work and returns with IRQs off.  This function is
diff --git a/arch/x86/include/asm/ia32.h b/arch/x86/include/asm/ia32.h
index 2c5f7861d373c..3486ad645be4b 100644
--- a/arch/x86/include/asm/ia32.h
+++ b/arch/x86/include/asm/ia32.h
@@ -68,6 +68,20 @@ extern void ia32_pick_mmap_layout(struct mm_struct *mm);
 
 #endif
 
-#endif /* !CONFIG_IA32_SUPPORT */
+extern bool __ia32_enabled;
+
+static inline bool ia32_enabled(void)
+{
+	return __ia32_enabled;
+}
+
+#else /* !CONFIG_IA32_SUPPORT */
+
+static inline bool ia32_enabled(void)
+{
+	return IS_ENABLED(CONFIG_X86_32);
+}
+
+#endif
 
 #endif /* _ASM_X86_IA32_H */
-- 
2.34.1